The present invention generally relates to the field of mechanical resonator sensors and methods and more particularly to the field of mechanical resonator sensors and methods for monitoring beds of particles and other multi-phase systems having particles. Such mechanical resonator sensors are suitable for implementation in fluidized bed systems including fluidized bed polymerization reactor systems for functions such as property characterization, process monitoring, and process control. The present invention relates, in preferred embodiments, to devices and methods adapted for use in open and/or closed fluidized bed systems such as fluidized bed polymerization reactor systems and related reaction and recirculating fluid systems (e.g., reaction chamber, velocity reduction zone, recycle line, etc.). The present invention relates, in particularly preferred embodiments, to the field of fluid sensor devices and methods involving a mechanical resonator sensor such as a flexural resonator sensor.
Effective approaches for measuring characteristics of fluids using mechanical resonators are disclosed in U.S. Pat. Nos. 6,401,519; 6,393,895; 6,336,353; 6,182,499; 6,494,079 and EP 0943091 B1, each of which are incorporated by reference herein for all purposes. See also, Matsiev, “Application of Flexural Mechanical Resonators to Simultaneous Measurements of Liquid Density and Viscosity,” IEEE International Ultrasonics Symposium, Oct. 17-20, 1999, Lake Tahoe, Nev., which is also incorporated by reference herein for all purposes. The use of a quartz oscillator in a sensor has been described as well in U.S. Pat. Nos. 6,223,589 and 5,741,961, and in Hammond, et al., “An Acoustic Automotive Engine Oil Quality Sensor”, Proceedings of the 1997 IEEE International Frequency Control Symposium, IEEE Catalog No. 97CH36016, pp. 72-80, May 28-30, 1997.
The use of other types of sensors is also known in the art. For example, the use of acoustic sensors has been addressed in applications such as viscosity measurement in J. W. Grate, et al, Anal. Chem. 65, 940A-948A (1993)); “Viscosity and Density Sensing with Ultrasonic Plate Waves”, B. A. Martin, S. W. Wenzel, and R. M. White, Sensors and Actuators, A21-A23 (1990), 704-708; “Preparation of chemically etched piezoelectric resonators for density meters and viscometers”, S. Trolier, Q. C. Xu, R. E. Newnham, Mat. Res. Bull. 22, 1267-74 (1987); “On-line Sensor for Density and Viscosity Measurement of a Liquid or Slurry for Process Control in the Food Industry”, Margaret S. Greenwood, Ph.D. James R. Skorpik, Judith Ann Bamberger, P. E. Sixth Conference on Food Engineering, 1999 AIChE Annual Meeting, Dallas, Tex.; U.S. Pat. Nos. 5,708,191; 5,886,250; 6,082,180; 6,082,181; and 6,311,549; and “Micromachined viscosity sensor for real-time polymerization monitoring”, O. Brand, J. M. English, S. A. Bidstrup, M. G. Allen, Transducers '97, 121-124 (1997). See also, U.S. Pat. No. 5,586,445 (“Low Refrigerant Charge Detection Using a Combined Pressure/Temperature Sensor”).
Notwithstanding the above, there remains a need in the art for alternative or improved sensor devices and methods for efficiently evaluating particles and multi-phase systems containing particles in beds and fluidized bed systems, including for example in fluidized bed polymerization reactor systems. Examples in which such a need exists include those fluidized bed systems used in connection with the petroleum, chemical, pharmaceutical, healthcare, environmental, military, aerospace, construction, heating, ventilating, air-conditioning, refrigeration, food, and transportation industries. In particular, there remains a need in the art for a cost-effective approach for monitoring particle and fluidized bed dynamics and properties in fluidized bed polymerization reactor systems, such as the systems disclosed in U.S. Pat. Nos. 5,317,036 and 6,689,847, each of which is incorporated by reference herein for all purposes. Because fluidized bed systems are often closed, it is difficult to monitor definable parameters and changes in condition, e.g., how the bed and its components are presently behaving and reacting to process changes, the level of product in the vessel, etc. This in turn makes it difficult to anticipate problems such as fluidized bed collapse, agglomeration, and sheeting.
Fluidized bed collapse can generally have undesirable consequences. In a fluidized bed polymerization reactor system, fluidized bed collapse is a very costly occurrence, both in terms of production time lost, and often the need to physically remove agglomerations from the reactor system before the fluidized bed can be reinitiated. Thus, it would be desirable to detect, in situ, characteristics of the fluidized bed that could indicate a likelihood of bed collapse.
Another issue requiring attention is how to ensure uniform fluidization of the bed. Uniform fluidization in a fluidized bed polymerization reactor system is important for many reasons, among them reaction efficiency, and avoidance of overheating. Because polymerization reactions are typically exothermic, heat transfer out of the reactor is critical to avoid such things as particle agglomeration and runaway reactions. Non-uniform fluidization of the bed can create “hot spots,” which in turn can cause the newly-formed polymer particles to become tacky due to elevated temperatures. The tackiness can cause particle agglomeration, and more devastatingly, sheeting. In agglomeration, the particles stick together, forming agglomerated particles that affect fluid flow and may be difficult to remove from the system. In sheeting, the tacky particles gather on a surface of the reactor system, such as the wall of the reactor vessel, forming a sheet of polymer particles. Sheeting is particularly malicious in that a sheet falling from the reactor wall can damage system components such as sensors as well as disrupt fluid flow resulting in collapse of the fluidized bed. When sheeting is finally detected, often after the damage is done, the reactor system must be stopped and the sheeting physically removed. Again, the lost production time is very costly. Thermocouples are used as sensors in the fluidized bed polymerization industry and are effective at detecting the local temperature of a gas flowing near the reactor wall, however, they provide very little indication of a uniformity of the bed. Nor do they provide any indication of a tackiness of the particles or if sheeting is occurring. Thus, it would be desirable to detect, in situ, characteristics of the fluidized bed and/or particles therein that could indicate a uniformity of the bed as well as provide an early indication of a likelihood or occurrence of particle agglomeration and sheeting.